Analysis of the variability of canopy resistance over a desert steppe site in Inner Mongolia, China

Canopy resistance substantially affects the partitioning of available energy over vegetated surfaces. This study analyzed the variability of canopy resistance and associated driving environmental factors over a desert steppe site in Inner Mongolia, China, through the use of eddy-flux and meteorological data collected from 2008 to 2010. Distinct seasonal and interannual variabilities in canopy resistance were identified within those three years, and these variabilities were controlled primarily by precipitation. Strong interannual variability was found in vapor pressure deficit (VPD), similar to that of air temperature. Based on the principal component regression method, the analysis of the relative contribution of five major environmental factors [soil-water content (SWC), leaf-area index (LAI), photosynthetically active radiation (K p), VPD, and air temperature] to canopy resistance showed that the canopy-resistance variation was most responsive to SWC (with > 35% contribution), followed by LAI, especially for water-stressed soil conditions (> 20% influence), and VPD (consistently with an influence of approximately 20%). Canopy-resistance variations did not respond to K p due to the small interannual variability in K p during the three years. These analyses were used to develop a new exponential function of water stress for the widely used Jarvis scheme, which substantially improved the calculation of canopy resistance and latent heat fluxes, especially for moist and wet soils, and effectively reduced the high bias in evaporation estimated by the original Jarvis scheme. This study highlighted the important control of canopy resistance on plant evaporation and growth for the investigated desert steppe site with a relatively low LAI.